U.S. Pat. No. 4,774,581 entitled "Television Picture Zoom System," which is hereby incorporated by reference, relates to circuitry that produces a magnified image representing a portion of a video frame from a conventional television signal. The portion of the field which is magnified, hereinafter referred to as the source portion, is determined by signals provided via viewer controls. These controls define the upper left corner of the image, in terms of horizontal and vertical pixel positions in the unmagnified field, and a magnification factor to be used to produce the magnified display.
In this referenced patent, the upper left corner of the source portion is used as a reference point when the magnification factor is changed or when the source portion is panned around the unmagnified field. However, this may not be the best reference point. Intuitively, a viewer manipulating the controls would expect the center of the image to be the reference point since this is the reference used by mechanical zoom apparatus such as photographic zoom lenses. If, for example, the upper left corner of a source portion were used as the reference point, an object of interest in the center of the picture may be moved out of the display when the magnification factor is increased.
From the point of view of the circuit designer, it is more convenient to provide the pixel positions of the upper left corner of the portion of the image to be magnified to the zoom and pan system. If, for example, the portion of the image being magnified were near an edge of the frame and the magnification factor were reduced, maintaining a constant center position would cause portions of the video signal outside of the active video region (i.e. the horizontal or vertical blanking intervals) to be included in the magnified display. Moreover, it is easier to determine the allowable limits of the source portion during a pan operation when the upper left corner is used as the reference point than when the image center is used as the reference point.